1. Technical Field
The present invention relates to image processing devices for printing, and relates more particularly to image processing devices that can adjust black separately from chromatic colors and apply density adjustment that is capable of controlling the amount of color material according to the density to suitably meet user needs for density adjustment.
2. Related Art
Inkjet printers and other types of printers are used for a wide range of applications. Such printers print by depositing color material onto paper or other print medium and forming dots. This type of printing process requires image processing to convert the image data to be printed to data expressing whether or not dots are formed, and this printing process is performed by a printer driver or printer controller on the host device.
Image processing includes a process to adjust density based on user needs. Density adjustment methods according to the related art include methods of adjusting the density of each color in the image data to the density corresponding to a set density value as described in Japanese Unexamined Patent Appl. Pub. JP-A-2009-141941. Under color removal (UCR) is also commonly used for density adjustment as described in Japanese Unexamined Patent Appl. Pubs. JP-A-2006-279922 and JP-A-2008-205964.
However, if the color expression of the processed data differs from the color expression of the color material used by the printer, the increase or decrease in density by the density adjustment process does not translate directly to the same increase or decrease in the amount of color material deposited on the print medium, and the increase or decrease in density may actually be reversed in some cases, when applying density adjustment to image data as described above. For example, when CMYK (cyan, magenta, yellow, black) color material is used by the printer and the image data is expressed in the RGB (red, green, blue) color space, the change in the amount of color material due to the density increase or decrease will produce different results depending on the design of the conversion process because the conversion from RGB to CMYK is not uniform. More specifically, if the adjustment process lowers the density of RGB (0, 0, 0) (that is, the color black in data in which the density of each color is expressed in 256 levels) 10%, the resulting values are (20, 20, 20). If these values are then converted to CMYK data (data representing the surface coverage of color material per unit area proportional to the amount of color material), the resulting values are (10%, 10%, 10%, 75%). However, converting the original RGB value of (0, 0, 0) directly to the CMYK space results in (10%, 10%, 10%, 75%), and even though the result of density adjustment is a density decrease of 10%, the amount of color material increases ((100% (K)->105% (CMYK total)).
The purpose of reducing density through density adjustment includes, for example, preventing bleed-through and bleeding in the paper (print medium), and reducing the cost of ink (color material) used for printing. This requires reducing the amount of color material that is physically deposited in addition to reducing the visible density, but the actual reduction in color material in the above situation is difficult to determine and adjustment is difficult.
Another reason for increasing density in density adjustment is to solve the problem of so-called “insufficient fill” in the printout, but this requires suitably increasing the amount of color material according to the increase in density, and is not possible under the conditions described above.
Under color removal methods such as described above are methods of removing and replacing color ink with black ink. However, such methods are difficult to use when adjusting black separately from other colors is desirable.
For example, reducing total ink deposition by a specified amount without reducing the quality of black text and barcodes and without increasing the amount of black ink is difficult using such UCR methods.